For optimum efficiency, a telecommunication line driver should contain no dissipative elements in series with its output. Prior to the introduction of synthetic drivers, this meant not matching the driver's output impedance to that of the line. Until recently, this has not been a problem in a T1 DSX environment, as the terminating end was well matched to the line impedance, and the line impedance was homogeneous as a result of strict control of the type of cable being used. However, as customers of telecommunication equipment have become less careful about controlling the impedance of DSX-1 cross-connect cables in central office installations, the line impedance mismatch problem cannot be ignored.
As shown diagrammatically in the ‘classical’ circuit architecture of FIG. 1, the driver-to-line impedance mismatch problem has been conventionally addressed by terminating the output 11 of a line driver amplifier 10 with a line-coupling output resistor 13, whose value Ro is set equal to the impedance Rt (e.g., 100 ohms or more) of the line 15. Unfortunately, the power utilization efficiency is very poor, as the resulting voltage divider formed by the output resistor 13 and the line impedance 15 dissipates (wastes) half of the line driver's output power in the output impedance 13.
One approach to reducing this power dissipation problem is to synthesize the driver's output impedance in accordance with the value of a much smaller resistor, and simulate the line-matching impedance by the judicious use of positive feedback. A typical set of synthetic impedance parameters may reduce the value of the output resistor to only a fraction (e.g., one-fourth) of its normal value, and (electronically) synthesize the remaining portion (e.g., three-fourths) of the output impedance. In the case of a one-fourth/three-fourths split, the output voltage swing of the driver amplifier can be reduced from twice to only five-fourths the desired output voltage. In the ideal case, the amplifier power supply rails can be reduced to five-eighths of the supply differential for a classic driver.
It is not uncommon for circuit implementations of synthetic output impedance drivers to employ some form of relatively complex cross-coupling network or other feedback arrangements. As shown in the circuit diagram of FIG. 2, a synthesized impedance driver may be modelled as containing a voltage source Vm and an output impedance formed of two parts: 1—a synthetic resistance Rsyn, and 2—a physical output resistance Rphy. The voltage swing produced by the amplifier is Va, and the voltage swing across a load resistance RL (relative to a ground reference) is VL. For the case of reducing the physical resistor to one-fourth of its normal value, then Rphy=25% of RL and Rsyn=75% of RL. Therefore, the driver voltage source Vm must be able to provide a voltage swing of 2VL. As pointed out above, the actual voltage Va at the output node of the amplifier need only swing to 5/4 VL.
In some applications, Va must swing to a voltage greater than 5/4 VL. For example, if the value of the load resistor RL is increased substantially, Va must swing to almost 2VL. While this may cause clipping, it has the benefit of reduced output current with the increased output voltage swing. As such, a synthetic impedance line driver might also be required to swing to 2VL, just as in the case of the classical line driver. In this case, however, the synthetic driver is delivering zero or minimum current, while the classic driver is delivering its maximum output current.
The U.S. Pat. to Joffe et al, No. 5,856,758, assigned to the assignee of the present application and the disclosure of which is incorporated herein, describes an improved efficiency, positive feedback-based line driver circuit architecture that reduces the required output signal amplitude excursion required for driving the line, enhances linearity, and allows the line to be driven from amplifiers which run with a lower supply voltage, and thus results in lower power dissipation. In the Joffe et al patent, the value of the output resistor is dramatically reduced so as to enable the amount of power dissipated across the driver's output resistor to be much smaller than the one-half value of a classical driver; yet, due to positive feedback, the effective electrical output impedance seen at the line driver's output node is matched to the line impedance.